Measurement uncertainty and temporal resolution of Doppler global velocimetry using laser frequency modulation

A Doppler global velocimetry (DGV) measurement technique with a sinusoidal laser frequency modulation is presented for measuring velocity fields in fluid flows. A cesium absorption cell is used for the conversion of the Doppler shift frequency into a change in light intensity, which can be measured by a fiber coupled avalanche photo diode array. Because of a harmonic analysis of the detector element signals, no errors due to detector offset drifts occur and no reference detector array is necessary for measuring the scattered light power. Hence, large errors such as image misalignment errors and beam split errors are eliminated. Furthermore, the measurement system is also capable of achieving high measurement rates up to the modulation frequency (100 kHz) and thus opens new perspectives to multiple point investigations of instationary flows, e.g., for turbulence analysis. A fundamental measurement uncertainty analysis based on the theory of Cramér and Rao is given and validated by experimental results. The current relation between time resolution and measurement uncertainty, as well as further optimization strategies, are discussed.     

Directional light scanning laser ophthalmoscope

The cone photoreceptor mosaic of the living human eye has in a limited number of cases been imaged without the use of wavefront-correction techniques. To accomplish this, the directionality of the photoreceptors, as manifested by their waveguiding properties, may be used to advantage. In the present paper we provide a model of our recently proposed directional light scanning laser ophthalmoscope [Opt. Lett. 29, 968 (2004)] together with a detailed numerical analysis of the device. The outcome is compared with experimental results.     

Analog sensor design proposal for laser Doppler velocimetry

Laser Doppler velocimetry (LDV) has been widely used for many years in fluid mechanics to measure particle velocity. However, in most applications, i.e., in industrial processes, such a system is often too expensive. This paper discusses a technique based on the use of an analog phase-locked loop and an analog integrator system for processing laser Doppler velocimeter data to infer particle velocity. This method appears to be suitable for designing low-cost integrated LDV sensors. A SIMULINK program has been written in order to validate the method for velocities in the 10-80 mm/s range. Finally, the performance of the estimator is illustrated by Monte-Carlo simulations obtained from synthesized Doppler signal.     

频率调制多普勒全场测速实验系统测试

频率调制多普勒全场测速技术是一种基于分子滤波和多普勒频移现象的流场速度测量方法,在高速、超高速及大尺度风洞流场测量方面潜力巨大.我们设计开发了采用CCD相机作为接收探头的FM-DGV实验系统,该系统主要包括激光器、片光光学系统、碘分子滤波器、图像采集相机、频率监测单元等.基于该系统进行了谐波幅值比和转盘线速度测试实验.实验结果表明,该实验系统工作正常,速度测量误差最大值小于2m/s.     

Perturbation analysis of frequency shifts in an electroelastic body under biasing fields

We analyze the eigenvalue problem associated with small-amplitude vibrations superposed on finite-biasing fields in an electroelastic body. The widely used first-order perturbation integral by Tiersten is generalized in two different ways: a second-order perturbation analysis is given when the biasing fields are not infinitesimal and their second order effects need to be considered; a first-order perturbation analysis is given when an eigenvalue is associated with more than one eigenvector (a degenerate eigenvalue).     

Glass-fiber self-mixing intra-arterial laser Doppler velocimetry: signal stability and feedback analysis

We have developed a blood velocimeter based on the principle of self-mixing in a semiconductor laser diode through an optical fiber. The intensity of the light is modulated by feedback from moving scattering particles that contain the Doppler-shift frequency. Upon feedback the characteristics of the laser diode change. The threshold current decreases, and an instable region may become present above the new threshold. The amplitude of the Doppler signal turns out to be related to the difference in intensity between situations with and without feedback. This amplitude is highest just above feedback. The suppression of reflection from the glass-fiber facets is of paramount importance in the obtaining of a higher signal-to-noise ratio. Using an optical stabilization of the feedback, we optimized the performance of the laser-fiber system and the Doppler modulation depth and clarified its behavior with a suitable physical model. We also investigated the effect of the finite coherence length of the laser. We tested the efficiency of the self-mixing velocimeter in vivo with the optical glass fiber inserted in the artery with endoscopic catheters, both in upstream and in downstream blood flow conditions. For the latter we used a special side-reflecting device solution for the fiber facet to allow downstream measurements.     

Determining the equation of state of highly plasticised metals from boundary velocimetry: part I

Assuming that a highly plasticised metal has an equation of state that relates pressure to density, an inverse problem is set up to determine this equation of state from boundary velocity measurements. A transformation into the hodograph plane then leads to an overdetermined Goursat problem, which is used to find the polytropic equation of state that is in best agreement with the measurements.     

Self-mixing feedback in a laser diode for intra-arterial optical blood velocimetry

Intra-arterial measurements of the velocity and the average flow of red-blood cells were investigated by means of a fiber-coupled laser Doppler velocimeter based on the self-mixing effect. The velocity of the red cells was calculated from the frequency of the signal that occurs when light, scattered back from a moving object in front of a fiber into a laser-diode cavity, interferes with the laser cavity's proper mode. These fluctuations occur at the Doppler frequency. The signal was obtained from the photodiode that is present in the laser diode's housing. Temperature control and stabilization of the diode cavity were introduced to reduce the light-intensity fluctuation that is due to mode hopping of the diode. The velocimeter was calibrated with a rotating disk covered with white paper (nonlinearity of 2.6% for velocities up to 0.4 m/s) and tested in vitro as a fluid velocimeter. The velocimeter was used in in vivo tests on the iliac artery of a 35-kg pig and on the arteria pulmonaris of a healthy calf. The optical fiber was placed in the iliac artery by a basket catheter 4 cm proximal to the bifurcation of the femoral artery. The average arterial blood flow velocity of the red cells were measured upstream and downstream. A special cleaving procedure for the fiber tip in downstream measurement is reported. Blood-velocity measurement is compared with values generated by an ultrasound flowmeter, and a difference of less than 9% is found.     

基于激光诊断的分子标记示踪测速技术研究进展

随着飞行器速度的不断提升,高超声速气流流动理论、高超声速燃烧流场分析、飞行器空气动力学特性等研究的重要性日益凸显。目前常规的速度测量方法在面对高超声速复杂流场环境时受到的限制越来越明显,需要研究新的技术以满足流场内精确速度测量需求。分子标记示踪测速技术因其非侵入、无跟随性限制等优势正在成为研究热点。本文阐述了纳秒激光分子标记示踪测速技术、飞秒激光分子标记示踪测速技术的基本原理、主要参数和工作特点,分析了这些技术在测量中所面对的挑战,并对其在科学及工程领域中的应用前景展开了讨论,为推动高超声速复杂流场环境速度测量技术发展提供借鉴。     

薄膜体声波谐振器应力负载效应摄动分析

薄膜体声波谐振器(FBAR)力学传感器有很大的应用潜力,但其敏感机理——应力负载效应尚不能被准确描述。为准确描述应力负载效应,预测FBAR力学传感器的频率灵敏度,提出一种摄动与有限元联合求解方法,并利用该方法计算FBAR微加速度计的频率-加速度灵敏度。首先,在COMSOL有限元软件中计算FBAR微加速度计在加速度下其压电层AlN的平均偏置应力;接着,在COMSOL中计算单个FBAR的谐振频率与相应的振型;最后,将有限元的计算数据和AlN的材料常数代入摄动积分公式中,得到FBAR微加速度计的频率-加速度灵敏度约为–98.879 kHz/g,与文献报道的实验结果–100 kHz/g相吻合,验证方法的可行性。     

To simulate blackbody radiation frequency shift in cesium fountain frequency standard with CO2 laser.

An experimental scheme to simulate the Blackbody radiation (BBR) frequency shift with carbon dioxide (CO2) laser in cesium fountain frequency standard is proposed. The alternative (ac) electric field from a CO2 laser with 10.6 microm wavelength causes the similar ac Stark shift of the clock transition frequency, but with much higher intensity than BBR power intensity at about room temperature. This scheme provides a simple, new method for measuring the BBR frequency shift in fountain clock. It provides the possibility to explain the difference of BBR shifts in Cs fountain clocks between two measurement methods.     

Quasi-phase-matched intracavity laser frequency summation

The work presents a theoretical analysis of quasi-phase-matched intracavity interaction in the constant-intensity approximation at frequencies summing with simultaneous regard for the losses and phases of interacting waves. An analytical expression for optimum correlation between interacting waves has been received. It is shown that, by the choice of optimum values of phase mismatch, pump intensity, and phase relationship, it is possible considerably to increase conversion efficiency in comparison with the noncavity case. The numerical estimation of expected conversion efficacy in conditions of an experiment is presented.     

偏场下薄膜体声波谐振器频率偏移的摄动分析

为准确预测测量力、热场的薄膜体声波谐振器(FBAR)传感器的灵敏度,采用叠加于有限偏场之上的小增量场理论描述,提出一种摄动与有限元联合求解方法。该方法利用COMSOL有限元软件计算FBAR传感器受外界载荷下其压电层AlN的平均偏置应力,进一步在COMSOL中计算FBAR的谐振频率与相应的振型,将有限元的计算数据代入摄动积分公式中,得到FBAR传感器的频率灵敏度。并以一个圆膜片FBAR为案例,介绍该方法用于计算圆膜片FBAR频率-集中力灵敏度的详细过程。采用摄动与有限元联合求解方法得到的频率灵敏度为41.3 MHz/N,与文献报道的实验结果 50 MHz/N接近,验证了方法的可行性。     

Lenticular array for spatial filtering velocimetry of laser speckles from solid surfaces

We present a low-cost optical design for the detection of speckle translation, which can provide measures of in-plane translation or the rotation of a solid structure. A nonspecular target surface is illuminated with coherent light. The scattered light is propagated through an optical arrangement that has been particularly designed for the type of mechanical measurand for which the sensor is intended. The dynamics of the speckle field that arise from the target surface are projected onto a lenticular array, constituting a narrow spatial bandpass filter for the speckle spectrum. The filter provides access to the full phase information of the temporal quasi-sinusoidal intensity output; thus differential arrangements of photodetectors can provide suppression of low-frequency oscillations and higher harmonics, and the direction of the speckle translation can be determined. The spatial filter of the sensor is characterized, and the precision of the sensor when it is integrated with an electronic zero-crossing-detection processor is investigated. The best measurement accuracy obtained at constant velocity is 1% at 1.6-mm translation; the relative standard deviation decreases with the square root of the distance traveled.     

Ultrasensitive laser measurements without tears

Several easily implemented devices for doing ultrasensitive optical measurements with noisy lasers are presented. They are all-electronic noise cancellation circuits that largely eliminate excess laser intensity noise as a source of measurement error and are widely applicable. Shot-noise-limited optical measurements can now easily be made at baseband with noisy lasers. These circuits are especially useful in situations where strong intermodulation effects exist, such as current-tuned diode laser spectroscopy. These inexpensive devices (parts cost approximately $10) can be optimized for particular applications such as wideband or differential measurements. Although they cannot eliminate phase noise effects, they can reduce amplitude noise by 55-70 dB or more, even in unattended operation, and usually achieve the shot-noise limit. With 1-Hz signal-to-noise ratios of 150-160 dB, they allow performance equal or superior to a complex heterodyne system in many cases, while using much simpler dual-beam or homodyne approaches. Although these devices are related to earlier differential and ratiometric techniques, their noise cancellation performance is much better. They work well at modulation frequencies from dc to several megahertz and should be extensible to approximately 100 MHz. The circuits work by subtracting photocurrents directly, with feedback applied outside the signal path to continuously adjust the subtraction for perfect balance; thus the excess noise and spurious modulation ideally cancel at all frequencies, leaving only the shot noise. The noise cancellation bandwidth is independent of the feedback bandwidth; it depends only on the speeds of the photodiodes and of the bipolar junction transistors used. Two noise-canceled outputs are available; one is a high-pass filtered voltage proportional to the signal photocurrent and the other is a low-pass filtered voltage related to the log ratio of the signal and comparison photocurrents. For reasonable current densities, the noise floors of the outputs depend only on the shot noise of the signal beam. Four variations on the basic circuit are presented: low noise floor, high cancellation, differential high power, and ratio-only. Emphasis is placed on the detailed operation and design considerations, especially performance extension by compensation of the nonideal character of system components. Experience has shown that some applications advice is required by most users, so that is provided as well.     

To simulate blackbody radiation frequency shift in cesium fountain frequency standard with CO/sub 2/ laser

An experimental scheme to simulate the blackbody radiation (BBR) frequency shift with carbon dioxide (CO/sub 2/) laser in cesium fountain frequency standard is proposed. The alternative (ac) electric field from a CO/sub 2/ laser with 10.6 /spl mu/m wavelength causes the similar ac Stark shift of the clock transition frequency, but with much higher intensity than BBR power intensity at about room temperature. This scheme provides a simple, new method for measuring the BBR frequency shift in fountain clock. It provides the possibility to explain the difference of BBR shifts in Cs fountain clocks between two measurement methods.